Under the support of the National Key Research and Development Program titled “High stability, full spectrum, high-efficiency solar cells materials exploration and device realization,” the School of Physical Science Technology’s Ning Zhijun Group explored Lead Sulfide (PbS) as the active layer and developed highly efficient inverted structural quantum dot (QDs) solar cells. Recently, their work has been published as “Highly Efficient Inverted Structural Quantum Dot Solar Cells” in Advanced Materials.

Due to their excellent light absorbance and emission characters, band-gap tunability, and solution processability, colloidal quantum dots (QDs) are largely regarded as an excellent optoelectronic material. For now, the highest efficiency of QD solar cell has reached 12%, but there is still a large gap to the ideal highest efficiency rate. Compared to normal structural cells, the depletion region of inverted structural solar cells is located on the luminescent side, which has the potential to improve carriers’ collection efficiency. Apart from that, the development of highly efficient inverted structural QD solar cells is important to the development of tandem solar cells. However, the inverted structural efficiency is far lower than normal structural cells, partly because it is hard to build an effective heterojunction between the hole transport layer and colloidal quantum dots active layer.

In their work, first author PhD candidate Ruili Wang and others took advantage of iodide capped n-type PbS QDs as the active layer, and p-type nickel oxide (NiO) as the hole transporting layer to form an effective heterojuction, which effectively increased the extraction and transportation of carriers. Based on further theoretical modeling, by inserting an 1,2-Ethanedithiol (EDT) capped PbS QDs layer between the hole transportation layer and n-type active layer as the buffer layer to construct a graded doping density structure, the surface carriers’ recombination is effectively reduced with an improved efficiency of 9.7%.

The construction of a highly efficient inverted structure provides another path to develop highly efficient QD solar cells. Other than that, inverted structures are widely used in solar cell structures. Highly efficient inverted structures provide an effective strategy to combine QD solar cells and other solar cells for the construction of full-spectrum tandem solar cells.

The first author of the paper is PhD candidate Wang Ruili from School of Physics and Science Technology, and co-first authors are undergraduates Wu Xun and Master’s candidate Xu Kaimin. Their research was supported by the Shanghai Science and Technology Research Foundation, the National Natural Science Foundation of China and the Shanghai Municipal Science and Technology Commission.

Read more at: http://onlinelibrary.wiley.com/doi/10.1002/adma.201704882/full

Figure. a) Schematic of non-graded and b) graded inverted QD solar cell. Characterizations for inverted QD solar cells. c) J–V curves under standard AM 1.5 light illumination (100 mW cm2) of the non-graded and graded devices. Both Jsc for two devices are higher than 27 Am cm2. e) EQE spectrum for graded structure device. d, f) Histogram of PEC with statistics based on 30 devices for the same batch. Average efficiencies are 7% and 8.5% for non-graded structure and graded structure, respectively.